A single San Diego-area poll moves our CA-48 forecast toward the middle

One new poll in California's 48th District shows a close race. SurveyUSA, polling for KGTV-TV and the San Diego Union-Tribune, surveyed 522 likely voters from August 27 to September 1 and found Democrat Marni von Wilpert at 45 percent and Republican Jim Desmond at 42 percent. Three points, in a sample that size, is not enough to separate them.

Our model has adjusted. It now gives von Wilpert a 26 in 100 chance of winning the seat, up from 26 percent's predecessor of 16 percent in the run completed at 8:32 a.m. ET on September 4. The mean margin moved from R+9.7 to R+6.3. The 5th-to-95th percentile range narrowed slightly on the Republican end and widened on the Democratic end, from R+25.8-to-D+6.2 to R+22.2-to-D+9.6. Desmond remains the favorite in our estimate, at 74 percent.

That is a real move, and it rests on a single survey. The forecast still sits meaningfully to the right of the poll itself, which is what a model built on more than one input does with one new data point: it takes some of it, not all of it. Our national generic ballot average, 25 polls over the past 45 days, stands at D+6.3, which is close to the district margin our model now expects here in the opposite direction.

For the House overall, our model gives Democrats a 93 percent chance of winning a majority, with a mean of 239.5 seats against the 218 needed for control. One caveat is ours to own: our simulation draws correlated error nationally, by census division, by state and by district, but not across demographically similar districts in different places, because we have no cluster data for it. That makes the House control number marginally firmer than a fuller treatment would leave it.